The present invention relates to a coated cemented carbide insert particularly useful as a cutting tool for the machining of cast iron at high speeds.
Cast iron materials may be divided into two main categories, namely grey cast iron and nodular cast iron. From machinability point of view these two materials are quite different. There are also a number of other cast iron materials having intermediate machinability properties, such as the newly developed compact graphite iron.
Grey cast irons have graphite flakes well distributed in the microstructure and are comparatively easy to machine. These flakes form short chips and provide a lubricating effect in the cutting zone. At high cutting speeds the cemented carbide inserts used in cutting tools for machining are mainly subjected to abrasive and diffusional wear.
Nodular cast irons are long chipping materials and their greater deformation resistance leads to a higher temperature level in the cutting zone of the cutting tool insert. This gives rise to excessive wear due to plastic deformation of the cutting edge of the cutting insert by creep.
U.S. Pat. No. 5,945,207 discloses a coated cutting insert particularly useful for the machining of cast iron parts by turning. It is exemplary of cemented carbide based tools useful for such applications and is recommended for use at cutting speeds of 200-300 m/min and 150-200 m/min, respectively when turning grey cast iron and nodular cast iron at a feed of 0.4 mm/rev.
For the machining of cast iron at higher speeds, Si3N4 based ceramic tools are normally used. The recommended cutting speeds when using tools of this ceramic material, at the same feed as above, are 400-700 m/min for turning of grey cast iron and 200-300 m/min of nodular cast iron. However, such tools suffer from brittleness and are more expensive to produce than corresponding coated cemented carbide tools. Therefore, it would be more cost effective if cemented carbide inserts could be used for machining, turning or milling a cast iron components at higher speeds when compared to prior art. Further, the use of cemented carbide based inserts instead of ceramic inserts decreases the risk of premature rupture and accordingly increases the possibility to estimate a useful life of the inserts.
U.S. Pat. No. 4,843,039 teaches how to produce cemented carbide bodies suitable for chip forming machining having a core containing eta phase, M6C (Co3W3C) and/or M12C (Co6W6C) embedded in normal alpha (WC)+beta (Co binder phase), said core being surrounded by a surface zone containing alpha and beta phase. The surface zone is free of eta phase and has a lower binder phase content than the nominal content of binder phase in the sintered body. The inner part of the surface zone situated nearest to the core has a content of binder phase greater than the nominal content of binder phase in the sintered body. Thus, the cemented carbide body obtained has a surface zone with comparatively low cobalt content, i.e. having a high resistance to creep deformation, followed by a zone with high Co content having a high ductility.
It is an object of this invention to provide a coated cutting tool particularly useful for the machining of cast iron parts by turning, milling or drilling at high speeds.
In one aspect, the present invention provides an article comprising a wear resistant coating applied to a cemented carbide body wherein:
the cemented carbide body comprises WC with an average grain size of 0.5-4 xcexcm, 3.5-9 wt-% Co and  less than 2 wt % carbides of Ta, Ti and Nb, said body further comprising a core containing finely distributed eta phase islands with a size of 1-15 xcexcm, the core containing 10-35 vol-% WC and Co binder phase, said body further comprising an intermediate zone 50-250 xcexcm thick and is essentially free of eta phase and with nominal Co-content, said body further comprising a 0-25 xcexcm thick surface zone free of eta phase with a Co content lower than the nominal Co-content of the body;
wherein the binder phase in the intermediate zone comprises a bimodal structure of smaller original eta phase islands and larger eta phase islands.
In another aspect, the present invention provides a method of making a coated cemented carbide body, the body comprising a cemented carbide of WC with an average grain size of 0.5-4 xcexcm, 3.5-9 wt-% Co and  less than 2 wt-% carbides of Ta, Ti and Nb and with a substoichiometric carbon content, the method comprising: sintering the body such that an eta phase containing structure is obtained in which the eta phase is finely distributed with a size of 1-15 xcexcm and a content of 10 vol-% to 35 vol-%, and subjecting the cemented carbide body to a gentle recarburisation such that the eta phase in a 50-350 xcexcm wide intermediate zone is transformed to WC+Co without essentially changing its Co-content.